It is well known to persons of ordinary skill in the art that electrical components and devices, by their operation, generate heat and are, by their nature, susceptible to damage by the elevated temperatures. Frequently, specific devices are provided with heat sinks, such as heat-conductive fins, to remove unwanted heat by radiation, conduction and convection to the environment.
Space limitations in the drive towards miniaturization have resulted in smaller space allowances for electronic components used in, for example, data processing equipment and peripherals, especially desk top and portable units. The electronic equipment is typically packaged in a cabinet or enclosure which protects the devices but frequently provides minimal room within for convection cooling. To provide air circulation, the cabinets, an arrangement known as "hard-mounting." The fans are generally known as "muffin fans" and typically comprise a propeller rotatably mounted within a frame on a hub containing the prime mover. Holes are typically disposed in the four corners of the frame for receiving bolts which secure the fan to a panel on the cabinet.
Hard-mounting can result in the vibrational excitation of the cabinet at one-per-revolution and blade-pass frequencies, and their respective harmonics, of the fan during operation as well as at the power line frequency and its harmonics. This structure-borne component of noise can be most undesirable for particular locations, such as when the electronic cabinets are within an office, and especially when several such fans are operating. Naturally, it is desirable to provide work and social environments which are free of unwanted acoustic noise. In normally quiet environments, such as law offices, banks, libraries and such, acoustic noise which emanates from the cooling fans used in computers and other business equipment can be quite annoying.
The noise which emanates from cooling fans originates from two very different causes: mechanical/electromagnetic forces or stresses and aerodynamic forces or pressures. Aerodynamic noises are caused by unsteady airflow in the vicinity of the fan. The unsteadiness results from discreet blades cutting through the air stream, from turbulence caused by sufficiently rapid flow of air over fluid boundaries such as fan rotor, fan case, fan motor, fan struts, and from other causes. Aerodynamic noise originates in the fluid at or near a fluid/solid boundary and is immediately radiated into the surrounding fluid, which is generally air. The sound energy continues to propagate through the fluid, reaching the exterior of the product, either directly (through fluid-borne paths entirely) or indirectly, transmitting through panels, covers, and the like, to the exterior. In the environments under consideration, aerodynamic noise typically is the least important.
Mechanical noise originates in the mechanical vibration of the fan, caused by unsteady electromagnetic forces, unsteady bearing forces, to some degree unsteady electromagnetic forces, unsteady bearing forces, to some degree unsteady aerodynamic forces, and other causes. This vibratory energy propagates through solid structures as audio frequency vibration (also known as structure-borne sound) and is only radiated into the surrounding fluid at fluid/solid interfaces. From there, it travels as fluid-borne sound to the exterior of the product, either directly (through fluid-borne pass entirely) or indirectly, transmitting through panels, covers, and the like to the exterior. In the absence of damping or vibration isolation, the mechanically or electromagnetically induced vibration propagates from its origin (in the fan) throughout the fan and to the supporting structure. A large area is energized by the vibration and, consequently, a large area radiates sound in the manner of a sounding or resonating board.
Mechanically induced noise from cooling fans used in computer and business equipment is generally controlled by isolating the fan vibrationally from its supporting structure. A typical prior art device is shown and described in U.S. Pat. No. 4,807,718 to Lotz, entitled "Acoustic Noise Control For Fans." As shown and described in Lotz, a standard muffin fan as commonly used for cooling in electronic equipment has mounting holes through which bolts are passed. The bolts, in turn, pass through rubber isolation grommets in the mounting hole surrounding a cooling opening in the side of the equipment case to be fastened by nuts. The rubber grommet fits within the mounting hole and includes flanged portions which extend above and below the equipment case so as to isolate the fan vibrationally from its support structure and the bolt from the fan case and thus reduces unwanted mechanically induced noise. Other prior art systems are also described in Lotz.
The use of cylindrical isolators (such as grommets) to reduce mechanically induced noise is widely accepted in the electronics industry. However, the use of the cylindrical isolators also requires the use of bolts which must pass through the fan case, the isolators, and the support structure or cabinet panel where they are ultimately fastened by nuts, resulting in numerous and complicated manufacturing steps. This manufacturing process often proves costly and cumbersome.
It is therefore a general object of the present invention to provide an acoustic isolation fastener which provides a reduction in mechanically induced noise and a method for attaching planar members using such a fastener which is simple and cost effective.
Another object of the present invention is to provide a resilient one-piece acoustic isolation fastener which can be used to fasten a device, such as a fan or a blower, to a surface on which the device is mounted.
A further object of the invention is to provide a resilient one-piece acoustic isolation fastener which can be used to fasten a small cooling device, such as a muffin fan, to a support surface, without the need for separate mounting bolts.
A still further object of the invention is to provide a method for attaching a device, such as a fan or blower, to a surface which is simple and cost-effective.